The fabrication of solar cell semiconductor devices typically involves the formation of metal contacts to a p-n junction device. The semiconductor material (e.g., silicon) absorbs light and generates electron and hole carriers which can then be separated by the p-n junction in the device. Majority carriers (e.g., electrons in n-type semiconductor material) are collected by the metal contacts which are formed to both the p-type and n-type material of the device. In commercially-produced screen-printed silicon solar cells, the n-type metal contacts are formed by screen printing and subsequently firing a silver paste in a grid pattern over the front (illuminated side) of the device. The p-type contact is formed by screen-printing the entire rear p-type surface of the device with an aluminium paste. This paste, when fired at temperatures of 780-870° C., forms a back-surface field (BSF) which reduces the recombination of the electron minority carriers (in p type material) at the silicon-metal interface and enables the collection of the hole majority carriers.
Screen-printed silicon solar cells have been industrially-produced for 25-30 years with continued improvements driving efficiencies towards 17-18% and 16-17% for mono-crystalline and multi-crystalline wafer substrates, respectively. One of the limitations of screen printed solar cells is that screen printed metal fingers formed on the illuminated surface of the solar cell effectively shade the cell and thus limit the generation of carriers in the cell. One way to address this issue has been to place all the metal contact regions on the rear surface of the cell. Such rear contact cells have been successfully manufactured, however although efficiencies as high as 24% have been achieved in a production environment, these rear contact technologies typically result in a higher cost per Watt of power generated than the less efficient commercially-produced screen-printed silicon solar cells. The higher cost of manufacture arises from the more complex processing required and the need to use higher quality silicon wafers to ensure that carriers generated towards the front (illuminated) surface of the solar cell can travel to a rear junction to be collected by the metal contacts.
Clearly what are required are new cost-effective manufacturing processes which can be applied to less-expensive, and potentially lower lifetime, silicon substrates in order to reduce the cell conversion costs of rear contact cells and make them more commercially competitive with existing screen-printed technology.